1. Field of the Invention
The present invention relates to a method and apparatus for a pressure measuring cell. More specifically, the present invention relates to a relative pressure measuring cell for measuring a pressure and having a base body with at least one base body electrode and with a membrane body connected to the base body to form a sensor chamber.
2. Description of the Related Art
The related art involves relative pressure sensors which are used to measure the differential pressure between the pressure in a measurement medium and the current atmospheric pressure. Such a relative pressure sensor is composed of a base body, which together with a measurement membrane joined to the latter at the margin forms a sensor chamber or pressure chamber. For the relative pressure measurement, the reference air is conducted into the sensor chamber through a vent opening in the base body, while the measurement pressure is applied to the surface of the membrane away from the sensor chamber. The deformation of the measurement membrane produced in this way is a measure of the relative pressure, which is converted into a measurement signal.
Ceramic capacitive pressure measuring cells are soldered or brazed together at the margin from a base-body sintered body and a membrane sintered body by means of a spacing holder, so that a sensor chamber is produced. The solder or active resin solder used, such as glass fit, serves as the spacing holder itself.
Before joining the base body and the membrane body, base body electrodes and a membrane electrode are applied to their surfaces that form the walls of the sensor chamber, usually by sputtering of tantalum.
Due to the supplying of the reference air through the vent opening, moisture gets into the sensor chamber, and when it drops below its dew point it may condense there and impair the function. Buildup of water influences the dielectric constant of the active electrode surfaces, resulting in an offset of the zero point of the pressure sensor.
In order to improve the tolerance with respect to moisture, it is proposed by EP 1 061 351 A1 to coat the entire inner surfaces of the sensor chamber of such a pressure measuring cell with a hydrophobic material, preferably making use of silanes. Since such coatings of silanes are organic, they have a limited temperature range of use. A further drawback is that such coatings on account of the temperature limitations can only be created after the joining of membrane and base body by means of a vacuum through the vent opening in the base body, i.e., a high manufacturing expense is required for this.
A similar method for further improving the tolerance with respect to humidity is described by DE 101 63 567 A1, in which the sensor chamber is likewise provided with a hydrophobic coating of silanes; but, this coating is created by means of a CVD (chemical vapor deposition) process, being deposited on the walls of the sensor chamber through the vent opening. Since this is likewise a vacuum method, the expense is very large and the result of a uniform layer deposition can only be checked with difficulty. Furthermore, the resulting layers of silane compounds are likewise not resistant to high temperature.
What is not appreciated by the prior art are the costs associated from protecting the sensor chambers of pressure measuring cells from the negative electrical effects from the buildup of water molecules within those chambers.
Accordingly, there is a need for an improved pressure measuring cell of the kind mentioned at the outset with a protective layer inside it, so that the negative electrical effects from buildup of water molecules are largely avoided, is simple to manufacture, and the aforementioned drawbacks in regard to the protective layer are avoided.